Many packaged products, particularly food and beverage products, are susceptible to deterioration due to oxygen and/or moisture absorption or loss through the wall of the package. Therefore, containers, either rigid, semirigid, flexible, lidded, collapsible, or a combination thereof, not only serve as a package for the product, but also help prevent the ingress of undesirable substances from the environment.
Atmospheric oxygen is one of the most reactive substances with products packaged in a container. Molecular oxygen (O2) is reduced to various highly reactive intermediate species by the addition of one to four electrons. The carbon-carbon double bonds present in virtually all foods and beverages are particularly susceptible to reaction with these reactive intermediate species. The resulting oxidation products adversely affect the performance, odor, and/or flavor of the product.
“Oxygen sensitive” materials, including foods, beverages, and pharmaceutical products, have special packaging requirements including preventing the ingress of exterior oxygen into the package and/or scavenging of oxygen that is present inside the package. In some cases, particularly in the orange juice and brewing industries, oxygen is removed from the product by vacuum, inert gas sparging, or both. However, it is difficult and expensive to remove the last traces of oxygen by these methods.
Containers made exclusively of glass or metal provide an excellent barrier both to egress of substances from the container and to ingress of substances from the environment. In most instances, gas permeation through a glass or metal container is negligible. Containers made of polymers, in whole or in part, generally do not possess the shelf life or barrier properties of glass or metal containers. Therefore, despite the great advantages of polymers, deficiencies restrict their use in containers.
The advantages of polymers include good mechanical, thermal, and optical properties, and an adaptability of container fabrication techniques that provides homogeneous, laminated, and/or coated containers. A further advantage of containers made from polymers include a light weight, reduced breakability, and low manufacturing cost.
Because of these advantages, the packaging industry is progressively shifting to plastic containers. This trend relates both to beverage containers, including carbonated beverages, and to food containers. In all these applications, insufficient barrier properties of the plastic material, particularly an insufficient capacity to prevent the passage of gases, e.g., oxygen and carbon dioxide, and vaporized liquids, e.g., water vapor, results in a reduced shelf life for products packaged in the plastic containers.
A number of solutions to overcome problems associated with plastic containers have been proposed. However, the proposed solutions failed to meet the commercially established requirements of low cost, in combination with high barrier properties, such that containers prepared from a plastic material can be practically employed. Examples of proposed solutions include:
a) laminates wherein two or more layers of a polymeric material are used, and the polymeric material in each layer optionally possesses a beneficial barrier property, for example, gas penetration, light penetration, or moisture penetration;
b) constructions wherein a metal, such as aluminum, either is positioned between layers of polymeric materials or forms the inner surface of the container; and
c) constructions wherein a layer of barrier material, other than a metal, is positioned between layers of a polymeric material or forms the inner surface of the container.
Other proposed solutions are those wherein plastic materials of different types are mixed, then molded to form containers. For example, it is known to manufacture containers of polymeric material containing a mixture of polyethylene terephthalate (PET) and polyamide. See, for example, e.g., U.S. Pat. Nos. 4,501,781; 4,837,115; 5,034,252; 5,258,233; 5,281,360; 5,641,825; and 5,759,653.
In particular, attempts to solve problems associated with polymeric, i.e., plastic, containers led to the widespread use of oxygen barriers and/or moisture barriers in packaging materials. Typical moisture barriers include polyethylene and polypropylene. Oxygen barriers include ethylene-vinyl alcohol copolymer (EVOH), polyvinyl alcohol (PVOH), nylon, and blends thereof. Vinylidene chloride/vinyl chloride copolymers and vinylidene chloride/methyl acrylate copolymers are used as both moisture and oxygen barriers.
It is difficult to manufacture commercially useful plastic containers solely from barrier materials because of their high cost, unstable structural properties, and other drawbacks. For example, EVOH has superior oxygen barrier properties, but suffers from moisture problems because of the plurality of hydroxyl groups on the polymer. Other barrier materials are sufficiently expensive such that containers manufactured solely from such materials is cost prohibitive. Accordingly, it became a common practice to manufacture multilayer structures whereby the amount of an expensive or sensitive barrier material is reduced to a thin layer, and an inexpensive polymer is positioned on one or both sides of the barrier layer as structural layers.
Although multilayer structures containing a barrier layer are less expensive and structurally stronger than a single layer of barrier material, such containers are more complicated to manufacture than single-layered containers. In addition, reducing the thickness of the barrier layer in a multilayer container often reduces the barrier properties of the container. Accordingly, in addition to multilayer containers having a barrier layer, there is a need in the art for a monolayer container having high barrier and structural properties, but without the high cost associated with a container prepared solely from a barrier material.
One material commonly used in packaging applications is polyethylene terephthalate resin, hereafter referred to as PET. PET has a number of advantageous properties for use in packaging applications, but PET does not possess the gas barrier properties that are required or desired in many applications. For example, although PET has good oxygen barrier properties for carbonated juices, PET has not been useful as a package material for other products, such as beer which rapidly loses flavor due to oxygen migration into the bottle, citrus products, tomato-based products, and aseptically packed meat. A packaging material with physical properties similar to PET is polyethylene naphthalate (PEN). PEN has barrier properties greater than PET, but PEN is considerably more expensive than PET.
Extremely impermeable polymers, such as copolymers of ethylene and vinyl alcohol, vinylidene chloride and vinyl chloride, and m-xylylenediamine and adipic acid (i.e., MXD6) exist. But because of practical or cost reasons, these copolymers typically are used as thin layers on or between PET layers or, in the case of MXD6, for blending with PET, in low weight percent amounts, to achieve an insignificant gas permeability. Also, using a xylylene group-containing polyamide resin with PET in amounts greater than 30% by weight causes the container to become a laminated foil structure thereby providing the possibility of exfoliation between the foil layers of the container.
From the foregoing, it is appreciated that the art requires an improved plastic, multilayered or monolayered container having excellent barrier properties for gases, such as oxygen and carbon dioxide. Products that can be satisfactorily packaged within such containers include, for example, beer (particularly lager beer), wine (particularly white wine), fruit juices, carbonated soft drinks, fruits, nuts, vegetables, meat products, baby foods, coffee, sauces, and dairy products. Multilayer and monolayer plastic containers having excellent barrier properties, and methods of preparing the same, are disclosed herein.